This application relates to a stretch forming apparatus, sometimes referred to as a stretch draw press, for forming drop hammer parts utilizing stretch wrap forming techniques. The apparatus and method described in this application combine in a novel manner techniques heretofore available only in distinctly separate machines and enables these functions to be carried out on a relatively low cost compact machine.
It is known that all metals and metal alloys become unusually ductile when stretched approximately 2 to 4%. In that state, referred to as a "yield state" or "yield range", such metals and metal alloys can not only be formed with about 1/3 the force normally required but also undergo a beneficial realignment of granular structure. Once the tension under which the forming takes place is released, completed parts exhibit increased tensile strength, freedom from internal stress and hightened shape retension. These phenomena are particularly useful in forming flat sheets of metal into complex shapes such as used in the aircraft and automobile manufacturing industry.
Prior art apparatus and techniques illustrative of the field in which this invention resides are illustrated in a publication of applicant entitled "Production Stretch-Draw Forming." Generally, stretch-draw forming can be broadly characterized as a method in which gripping jaws grasp opposite ends of a metal sheet and pull it to a minimum 2% stretch. After stretching, the sheet is pulled down over a bottom (male) die giving it a partial form. A top die mounted over the bottom die moves downwardly onto the stretched sheet completing the shape formation. Then, the gripper jaws move inwardly to release the tension on the sheet. The top die is raised away from the bottom die and the gripper jaws release the opposite sides of the metal sheet. Prior art devices of the type described are generally used in forming metal sheets having a relatively shallow formed shape. Other machines, such as radial draw forming machines, are used in forming deeper shapes, especially in elongated metal structures.
One problem heretofore encountered in stretch-draw forming large metal parts is the extent of vertical movement required to form a relatively deep formed shape in the metal sheet. Such machines are normally operated under semi-automatic as well as manual control and are always attended by at least one machine operator. The metal sheet, both before and after forming, must be accessible to the machine operators so that the formed sheet can be removed and a new, unformed sheet placed in the machine. This problem is not encountered in stretch-draw formers which apply only a relatively shallow form to the sheet. However, as the depth of the form increases, the vertical distance over which the sheet must be drawn increases substantially. Depending on the construction of the machine, the sheet is either elevated out of easy reach of the operator or opposite ends of the sheet are positioned, in their formed state, substantially below floor level.
The stretch forming apparatus and method disclosed in this application also permit the formation of parts which heretofore could be formed only by use of a drop hammer. This also results in the saving of substantial machine weight and floor space.
The stretch forming apparatus and method disclosed in this application allows conventional stretch-wrap forming of work pieces, available heretofore only on horizontal stretch-wrap forming machines, known generically as "swing arm machines", or on horizontal-type machines utilizing carriages and rail ways. The former has the disadvantages of requiring a large amount of floor space as well as being unsuited for combining stretch-wrapping with the secondary pressing action that achieves stretch-drawing since the drawing apparatus must necessarily be located in front of the machine, blocking access by the operators and rendering such a configuration impractical.
The latter, while allowing the stretch-wrapping to take place vertically and thus permitting an acceptable stretch-draw device to be employed, relies only on a rising die table to achieve stretch-wrap forming and thus the workpiece is necessarily raised well above floor level, over the operator's head, making it difficult to monitor and unload. The invention disclosed in this application relies on a new swinging arm concept to achieve wrapping in concert with a rising die table, mounting the machine in a vertical manner, thus permitting the stretch-drawing to take place vertically, above the machine out of the operator's way, and combining the two wrapping actions of arm swing and die table rise to keep the workpiece in a relatively centralized location, adding greatly to operator convenience.